Field of the Invention
The invention relates to image processing, and, in particular, to an image-capturing apparatus and an associated method for correcting flickers in single-shot multiple-exposure images.
Description of the Related Art
Fluorescent lighting can cause objectionable flicker in video images, resulting in degradation of the quality of the video images. Specifically, flicker occurs when a camera images an object under illumination of a flickering light source such as a fluorescent lamp. Fluorescent lighting has the property of responding very quickly to instantaneous changes in AC power line voltage. Thus, fluorescent light sources literally turn on and off on each half cycle of the AC power supply, resulting in periodic variation of the brightness of the fluorescent light source over time. The periodic brightness variation of a fluorescent light source is referred to as “fluorescent flicker”.
FIG. 1 is a diagram illustrating the light intensity of a fluorescent light source plotted over time. Fluorescent lighting acts as a rectifier of an AC power supply and the brightness or intensity of the light varies periodically at twice the frequency of the AC power supply. As shown in FIG. 1, when the AC power supply is assumed to be 50 Hz, a fluorescent light will turn on and off 100 times per second, resulting in a flicker frequency of 100 Hz. For a 60 Hz AC power supply, the flicker frequency of the fluorescent light will be 120 Hz. Although fluorescent flicker at 100 Hz or 120 Hz are not noticeable to the human eye, such flickering presents various problems for video or still image cameras.
However, if the video camera refresh rate has a frequency substantially different than the frequency of the AC power supply, then the flicker frequency of the fluorescent light does not have an integer multiple relationship with the refresh rate. Each captured field will end up sampling different portions of the periodic light variation cycles and therefore a different amount of light. This is illustrated in FIG. 1 where the flicker frequency is at 100 Hz while the refresh rate is at 60 Hz. As shown in FIG. 1, at each sampling field S1, S2, and S3, different amount of light is being sampled. When each image field samples different amount a light, the brightness of the resulting images will change from one field to another, resulting in observable and highly objectionable flickering in the video images.
In addition, line by line sampling is done one after another sequentially in an exposure image captured by an image sensor, and there is no clear time gap between line sampling. The flicker, i.e., horizontal banding, can be caused by the fact that each line of the exposure image is sampled at a slightly different time and that time span from the first line to the last line lasted more than one cycle of the AC power supply frequency which is not an integer multiple of the flicker frequency of the fluorescent light.